The invention relates to a carbon electrode having a conical or pyramidal tip, which is surrounded on its side by a raised edge.
Graphite electrodes are used in many different applications in industry. Examples of these are aluminum and steel production, electrolysis of salt melts, electrolytic decomposition of chemical compounds, thermal deposition reactions, arc welding, measuring instruments and many others.
One important application is the deposition of polysilicon by the Siemens process, in which highly pure elementary silicon is deposited from the gas phase on the surface of silicon rods. In this case, elemental silicon is deposited from a mixture of hydrogen and halosilanes, or a silicon compound containing hydrogen, in the gas phase, onto the surface of a thin silicon rod heated to from 900 to 1200° C. in a deposition reactor.
The silicon rods are held in the reactor by special electrodes, which generally consist of highly pure electrographite. Two thin rods with different voltage polarity on the electrode holders are respectively connected by a bridge at the other thin rod end to a closed electrical circuit. Electrical energy for heating the thin rods is supplied through the electrodes and their electrode holders. The diameter of the thin rods then grows. The electrode simultaneously grows, starting at its tip, into the rod foot of the silicon rods. After a desired setpoint diameter of the silicon rods has been reached, the deposition process is ended. The incandescent silicon rods are cooled and removed.
Particular importance is attached here to the material and the shape of the electrodes. They are used on the one hand for holding the thin rods, passing the flow of current into the silicon rod, but also for heat transfer as well as a secure support for the growing rod in the reactor. Since the trend is toward increasingly long and heavy rods and the rod pairs, which by now may weigh several hundred kilograms, are anchored in the reactor only by means of the electrodes, the choice of shape and material composition is in particular very important.
Also, depending on the subsequent use, very different requirements are placed on the silicon rods produced in this way and their deposition process—and therefore on the electrodes. If, for example, the polycrystalline silicon is subsequently used as crushed silicon for solar and electronic applications, the silicon rods must not collapse during or after the deposition process in the deposition reactor. Long and thick polycrystalline silicon rods increase the economic viability of the deposition process, but also the risk of collapse in the reactor.
Electrodes according to the prior art consist of a cylindrical main body in the lower part and a conical tip in the upper part. A hole for receiving the thin rod is formed in the conical tip. The lower end of the electrode is placed in a metal electrode holder, through which the current is supplied. Such electrodes are widely known and are used, for example in U.S. Pat. No. 5,284,640, for silicon deposition.
Graphite is mainly used as the material for the electrodes, since graphite is available with very high purity and is chemically inert under deposition conditions. Graphite furthermore has a very low electrical resistivity.
U.S. Pat. No. 6,639,192 describes a graphite electrode having a conventional shape. It consists of a cylindrical main body with a conical tip. The tip contains a hole for receiving the thin rod. The electrode is made in one piece and therefore from a material (here electrographite) with homogeneous material properties. It has, in particular, a very high specific thermal conductivity. A disadvantage with this embodiment is a high frequency of collapse before and during the deposition until the final diameter is reached.
DE 2328303 describes a cylindrical electrode without a tip. The carrier rod is fitted into a hole on a plane surface. This electrode shape has very high thermal dissipation even with a small rod diameter, owing to the solid cylindrical shape. So that the rods with a small diameter do not collapse during the deposition process, the electrode must have a low thermal dissipation, i.e. have a small diameter, and the electrode material must have a very low specific thermal conductivity. Thick rods, as are customary nowadays, cannot be deposited with this electrode shape since, owing to the small electrode diameter and the low specific thermal conductivity of the electrode material, the high energy required for thick rod diameters cannot be dissipated from the rod feet.
Graphite electrodes consisting of a plurality of layers are known from other fields. In these cases, however, the arrangement of different layers is intended to optimize chemical conversions. For example, U.S. Pat. No. 3,676,324 discloses a cylindrical graphite electrode which consists of a cylindrical inner part and a cylindrical outer part, the inner part having a very high electrical conductivity and the outer part being porous graphite. The purpose of these multiple layers is to avoid high voltage losses and obtain a high chemical conversion on the porous surface. A similar electrode having two different layers is known from GB 2135334, in which case the porous outer layer is used for the electrolytic production of fluorine.
A disadvantage with all electrodes known from the prior art is that they are relatively susceptible to cracking or flaking of material at the transition between the electrode and the silicon rod or in the silicon rod, in the vicinity of the electrode, so that they make the silicon rod unstable.
Collapsed batches entail great economical loss. If the silicon rods collapse, for example, the reactor wall may be damaged. The collapsed silicon rods are then contaminated by contact with the reactor wall and must be cleaned on the surface. Furthermore, collapsed batches can be removed from the reactor only with increased outlay. The surface of the silicon is then further contaminated.
It was an object of the invention to provide an electrode with which the frequency of collapse is reduced considerably compared with electrodes of conventional design.
Surprisingly, it has been found that a carbon electrode having a conical or pyramidal tip, which is surrounded on its side by a raised edge, has improved thermal dissipation during growth of the filament rod fastened in it and an improved distribution of the current density in relation to the rod thickness.